In general, a liquid crystal display (LCD) refers to a device that displays an image by injecting an liquid crystal material between a color filter substrate and a TFT-array substrate, the color filter substrate having a common electrode, a color filter and the like formed therein and the TFT-array substrate having a thin film transistor, pixel electrode and the like formed therein; changing arrangement of liquid crystal molecules by applying different potentials to the pixel electrode and the common electrode to form an electric field; and controlling transmittance of light through the orientation of the liquid crystal molecules.
Since this LCD has advantages that it is manufactured in small size and light weight, and driven at low power consumption, the LCD has received attention as an alternative image display system that may be used to solve the problems regarding to the cathode ray tube (CRT). Recently, the LCD has been used in almost all of the image display systems.
Meanwhile, since the LCD does not spontaneously emit light, a backlight unit is provided in a lower portion of the LCD panel to supply light to the LCD. In this case, the backlight unit is divided into an edge-type backlight unit and a direct-type backlight unit, depending on the position of a light source. FIGS. 1 and 2 are diagrams illustrating a configuration of a conventional LCD backlight unit. Here, FIG. 1 shows a direct-type backlight unit widely used in a large LCD, and FIG. 2 shows an edge-type backlight unit.
As shown in FIG. 1, optical films such as a diffusion plate 20, diffusion film 30 and 30 and a condensing film 40 are stacked on a light source 10 of the conventional direct-type backlight unit 1. The optical films function to shield a light source and diffuse and condense light to improve optical properties of the light emitted from the light source.
Meanwhile, the edge-type backlight unit 2 includes a light source 10, a light guide plate 50, diffusion film 30 and 30 and a condensing film 40, as shown in FIG. 2. Here, the light guide plate 50 functions to change a direction of light emitted from the light source 10 into a front direction (i.e. a direction toward a viewer).
The optical films of the backlight units may be configured more simply or more complicatedly, depending on the consumers demands, the desired use of the products, etc. For example, the lower diffusion film 30 or the upper diffusion film 30 may not be used, or two condensing films may be stacked and used, when necessary, in the backlight unit.
Among the optical films, the condensing film functions to concentrate light spreading in all directions within a viewing angle range in order to improve luminance of a screen. A prism film, a lenticular lens film or the like has been generally used as the condensing film. Recently, a lenticular lens film having a reflector layer formed in a lower surface thereof has been widely used.
FIG. 3 shows a conventional condensing film 100 including lenticular lenses and having a reflector layer formed in a lower surface thereof. As shown in FIG. 3, the condensing film 100 having a reflector layer formed therein includes lenticular lenses 120 sequentially arranged in an upper surface of a base member 110 made of a polyester (PET) film; and a reflector layer 130 formed in a lower surface of the base member 110. In this case, the reflector layer includes a reflective surface 150 and an opening 140. Here, only the light that may be condensed within the viewing angle range passes through the opening 140, and the other light reflects from the reflective surface 150.
Most of the light passed through the above-mentioned conventional condensing film is condensed and directed toward the front of a screen. Therefore, when a viewer watches the screen from sides or the top and bottom of the screen rather than the center of the screen, the brightness of the screen is gradually diminished since the intensity of light directed toward the viewer becomes lower, which leads to the lack of uniformity in brightness of the screen in up and down or left and right directions.
FIG. 4 is a graph illustrating the changes in luminance according to the viewing angle of the conventional condensing film. As shown in FIG. 4, it might be revealed that the luminance of the conventional condensing film is high in the vicinity of a viewing angle of 0 to 20° but is suddenly diminished out of the viewing angle range. The use of the conventional condensing film results in very narrow viewing angle range of a screen. This phenomenon becomes more serious when a thickness of the base member is similar to, or higher than a lens pitch of the lenticular lens. Therefore the emitted light in the condensing film should be distributed more moderately so as to improve a viewing angle range of LCD and display an image with uniform brightness.